How to Make It
In a large saucepan, whisk the mashed strawberries with the vinegar and lemon juice. Sprinkle the pectin evenly on top, then whisk until incorporated. Bring the mixture to a rolling boil over high heat. Whisk in the sugar, return to a boil and cook over high heat until glossy and the sugar dissolves, about 3 minutes. Remove from the heat and skim off any foam.
Pour the jelly into hot, sterilized canning jars to within 1/4 inch of the rims. Seal the jars, then submerge them in a pot of water and boil for 5 minutes. Carefully remove the jars and let cool. Check the jar lids to make sure they’ve fully sealed; refrigerate any jars with imperfect seals.